1. Field of the Invention
The present invention relates to a process of treating a selenium-containing liquid.
2. Background Art
In general, coals used in the coal fired power plants contain a very small amount of selenium. When the coals are burned in a coal fired power plant, the selenium in the coals is mixed into coal ashes captured by an electrostatic precipitator or into an absorbing solution (desulfurization slurry) from a wet flue gas desulfurization unit. Since the effluent standard value of selenium has been already set (0.1 mg/l) in Japan, the absorbing solution from the wet flue gas desulfurization unit may have to be treated to satisfy this standard even as desulphurization wastewater.
Selenium in the wastewater exists both as selenate (hexavalent selenic ion: SeO42−) and as selenite (quadrivalent selenic ion: SeO32−). In this case, the selenite can be treated by a general treatment process such as coagulation-sedimentation using iron or ion exchange method. On the other hand, the selenate cannot be treated by such a process. Therefore, there is known in the art a method of reducing the selenate into selenite or zero-valent metal selenium using granulating irons as a reducer, the selenite or zero-valent metal selenium being treated by the coagulation-sedimentation or other method (e.g., see a non-patent document, Yoshihiro Etoh and Others, Chemical Equipment of “Processing of New Effluent Standard Items, Selenium/Fluorine/Boron”, No. 8, 42 (2003)).